Light Detection And Ranging, abbreviated LADAR herein, is a surveying technology that measures distance by illuminating a target with a laser light and timing the return, and the term has been coined as an analogy for the previously well-known technology of radio detection and ranging (RADAR). Three dimensional (3D) imaging based on LADAR has been a rapidly expanding research field with approaches that capitalize on parallel readout architectures seeing significant advancement recently. For example, in flash LADAR systems, the target is broadly illuminated with a high energy pulsed laser and a receive aperture is used to image the target onto a fast focal-plane array (FPA) of optical detectors. The fast FPA typically consists of an array of avalanche photodiodes (APDs) coupled to an advanced read-out-integrated-circuit (ROIC), which also allows it to time resolve the return pulses. Several companies now manufacture fast FPAs operating either in linear or Geiger mode for short and long range flash LADAR respectively. Over the past decade, incoherent, direct-detect flash LADAR has been used for both terrestrial and airborne applications including mapping and autonomous navigation.
Synthetic aperture LADAR (SAL), distributed or sparse aperture imaging and holographic aperture LADAR (HAL) are coherent imaging techniques that coherently combine spatially and temporally diverse target returns to overcome the conventional diffraction limit. By recording individual estimates of the electric field with either a single optical detector or an optical detector array, and by considering the relative motion between the transceiver and the target, these field estimates can be synthesized or “stitched” in the pupil plane to produce enhanced resolution two dimensional (2D) imagery. The pupil plane is the plane in which light first impinges on the optics of a system, before any focusing of the impinging rays. To form 3D imagery, two or more baselines in an interferometric SAL system, or two or more wavelengths in a distributed aperture or HAL system have been proposed and demonstrated. The unresolved dimension is derived from the measured phase difference between the spatial or wavelength multiplexed 2D images. Because these techniques rely on a few synthetic wavelengths, targets with significant structure or discontinuities are poorly rendered. Recent work extended the number of discrete wavelengths to 256 to address these issues.